Vectorial polaritons in the quantum motion of a levitated nanosphere

The strong coupling between photons and bosonic excitations in matter produces hybrid quasiparticle states known as polaritons1–3. Their signature is the avoided crossing between the eigenfrequencies of the coupled system illustrated by the Jaynes–Cummings Hamiltonian4. It has been observed in quantum electrodynamics experiments based on atoms5,6, ions7, excitons8–10, spin ensembles11,12 and superconducting qubits13. In cavity optomechanics, polariton modes originate from the quantum-coherent coupling of a macroscopic mechanical vibration to the cavity radiation field14,15. Here we investigate polaritonic modes in the motion of an optically levitated nanosphere16–22 in the quantum-coherent coupling regime. The particle is trapped in a high vacuum by an optical tweezer and strongly coupled to a single cavity mode by coherent scattering of the tweezer photons23–27. The two-dimensional motion and optical cavity mode define an optomechanical system with three degrees of freedom. In the strong-coupling regime, we observe hybrid light–mechanical states with a vectorial nature. Our results pave the way towards protocols for quantum information transfer between photonic and phononic components and represent a step towards the demonstration of optomechanical entangled states at room temperature.